linux-kernel - Re: [PATCH bpf-next v2 4/9] bpf: Introduce load-acquire and store-release instructions

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Message-ID: <Z6Z5yLETaJ38TvqR@google.com>
Date: Fri, 7 Feb 2025 21:23:20 +0000
From: Peilin Ye <yepeilin@...gle.com>
To: Ilya Leoshkevich <iii@...ux.ibm.com>
Cc: bpf@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, bpf@...f.org,
	Xu Kuohai <xukuohai@...weicloud.com>,
	Eduard Zingerman <eddyz87@...il.com>,
	David Vernet <void@...ifault.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
	Jonathan Corbet <corbet@....net>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Puranjay Mohan <puranjay@...nel.org>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Quentin Monnet <qmo@...nel.org>,
	Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
	Ihor Solodrai <ihor.solodrai@...ux.dev>,
	Yingchi Long <longyingchi24s@....ac.cn>,
	Josh Don <joshdon@...gle.com>, Barret Rhoden <brho@...gle.com>,
	Neel Natu <neelnatu@...gle.com>,
	Benjamin Segall <bsegall@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 4/9] bpf: Introduce load-acquire and
 store-release instructions

Hi Ilya,

On Fri, Feb 07, 2025 at 12:28:43PM +0100, Ilya Leoshkevich wrote:
> Acked-by: Ilya Leoshkevich <iii@...ux.ibm.com>

Thanks!

> s390x has a strong memory model, and the regular load and store
> instructions are atomic as long as operand addresses are aligned.

I see.

> IIUC the verifier already enforces this unless BPF_F_ANY_ALIGNMENT
> is set, in which case whoever loaded the program is responsible for the
> consequences: memory accesses that happen to be unaligned would
> not trigger an exception, but they would not be atomic either.

The verifier rejects misaligned BPF_ATOMIC instructions since commit
ca36960211eb ("bpf: allow xadd only on aligned memory"), even if
BPF_F_ANY_ALIGNMENT is set - so this patch makes the verifier reject
misaligned load-acquires and store-releases, too, to keep the behaviour
consistent:

Specifically, check_atomic_load() calls check_load_mem() (and
check_atomic_store() calls check_store_reg()) with the
@strict_alignment_once argument equals true.  See also selftests
load_acquire_misaligned() and store_release_misaligned() in PATCH 8/9.

> So I can implement the new instructions as normal loads/stores after
> this series is merged.

That would be great!

Thanks,
Peilin Ye